The present invention relates to wireless communications, and more particularly to the sensing of wireless transmissions from a user of a spectral resource.
The radio spectrum is a limited resource that should be shared between many different types of equipment such as cellular, home network, broadcast, and military communication equipment. Historically, each part of the radio spectrum has been allocated to a certain use (called a “licensed” and/or “primary” use). This strategy has resulted in all applications/uses being disallowed on the allocated carrier frequency except for those applications included in the license agreement. In practice, this results in large parts of the radio spectrum being unused much of the time. For instance, in the Ultra-High Frequency (UHF) band, where TV broadcasts take place, large geographical areas are unused, mainly due to the large output power needed for such applications; this large output power compels a large reuse distance in order to minimize the risk of interference. An example of such geographical areas within Scandinavia is illustrated in FIG. 1. In FIG. 1, the shaded areas represent geographic locations in which a given carrier frequency is being used by a licensed user (e.g., by Broadcast TV). In the remaining areas, the so-called “white spaces”, the given carrier frequency is allocated to the licensed user but is not actually being used by that user.
In order to make better use of the licensed spectral resources, some countries will, in the future, allow unlicensed services (so called “secondary” uses) to take place in areas (called “white spaces”) in which the licensed (primary) user is not transmitting. However the primary user will always have priority for the use of the spectrum to the exclusion of others. Therefore, some sort of mechanism needs to be in place to ensure that the unlicensed users are not causing interference to the licensed user.
One mechanism is to install the unlicensed network in a geographical area where at least some parts of the licensed spectra are known to be unused.
However, even more use of the white space can be made if the non-interference mechanism adopts a detection strategy in which it operates on the licensed frequency (or frequencies) in the white space only so long as no licensed user transmissions are detected, and ceases such operation as soon as licensed user transmissions are detected. In this context, ceasing operation may mean ceasing all operation, or alternatively may mean ceasing operation only on those frequencies that are detected as being “in use”, and otherwise continuing to operate on other frequencies in the white space. The most straightforward sensor is a signature detector adapted to detect specific signatures transmitted from the licensed/primary user (typically implemented as a matched filer). An example of a white space system currently being standardized is IEEE 802.22. An overview of this system can be found in Cordeiro et al, “IEEE 802.22: An introduction to the First Wireless Standard based on Cognitive Radios”, Journal of Communications, Vol 1, No 1, April 2006.
Another consideration regarding the sensing of the licensed user's transmissions is placement of the sensors. When the secondary (e.g., unlicensed) use is for cellular telecommunications, one solution is to include the sensors in the base station of the mobile communication system. Sometimes, the base station's (or network's) own sensors do not provide enough information (e.g., information about the geographical positions of active white space transmitters) for the base stations to have a clear picture of white space spectrum availability as a function of geographical position. Without this information, it is difficult for a base station to use the white space fully. To compensate for this lack of information, it may be necessary to impose quite wide safety margins (for example with respect to frequency and/or power) in order to prevent the unlicensed user's interfering with the primary (licensed) user's use of white space frequencies.
As an alternative to locating the sensors at the base station, dedicated sensors can be distributed throughout the white space. However, this increases the complexity and cost of network implementation within the white space.
An alternative way of achieving a distributed set of sensors throughout the white space is to have sensing performed by each of the mobile terminals that are located within the white space. Each of these mobile terminals performs a sensing operation, and reports its findings to a main node (e.g., the mobile terminal's serving base station), the findings being either in the form of raw data or as some sort of processed data.
The sensing and measurement quality in a white space system needs to be sufficiently good in order to guarantee a lack of interference to the incumbent service. To achieve this, the sensing antenna must be able to detect signal energy arriving from any direction. As described in the Cordeiro et al. publication mentioned above, the white space system specifications require that the sensing antenna operate with an omni-directional pattern.
In practice, this requires a sensing antenna design that is distinct from the antenna(s) used by a mobile terminal for data communication with the base station and implies that an additional antenna unit needs to be included with the terminal. However, it is extremely difficult (and sometimes impossible) to obtain a reliable omni-directional pattern for an antenna embedded in the space-constrained industrial design of a handset, so the sensing antenna may have to be placed outside the unit. Even worse, in order to obtain sufficient sensing sensitivity with the omni-directional pattern (typically 0 dBi gain), the antenna itself may need to be placed outdoors.
The fact that a separate antenna is needed adds cost and design constraints to the terminal. It also complicates the use of existing terminal products in white space networks due to their lack of any antennas with omni-directional properties. Furthermore, placing the sensing antenna outdoors is not feasible in some usage scenarios.
There is therefore a need for an alternative sensing approach that permits mobile terminals to perform white space sensing tasks with high sensitivity without insisting on omni-directional antenna designs.